The easiest approach would be to specify the files in the StandardToolsFile and then configure the content editor to use the customized toolsfile instead of the embedded one. For further instructions on how to achieve this, I believe you might find the Editor CSS Styles
section of our online Documentation and Customizing the Sitefinity WYSIWYG Editor
guide useful. If you have any additional questions, please let us know.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items